In the operation of steam turbines for electric power plants, variations in load on the turbine are determined by controlling the steam flow through control or governor valves. Upstream of the governor valves, are throttle valves or stop valves. The throttle valves in some installations are used to control the turbine during start-up, at which time, the control or governor valves are fully open. In the turbine installations where stop valves are located upstream of the governor valves, the stop valves are fully opened during start-up; and the steam flow is controlled by the governor valve. In both types of installations, the throttle valves or the stop valves, as the case may be, are rapidly closed, to shut down the turbine in response to a contingency causing a turbine trip. However, once the turbine is placed in load operation, the throttle valves, or the stop valves are fully opened, with any variation in steam flow being controlled solely by the control or governor valves.
The period of uninterrupted load operation of the turbine, and therefore, the period of inactive throttle or stop valve operation, can be several months or more. Therefore, tests of the throttle or stop valves may be required, periodically, such as once a week, or even once a day, for example, in order to insure that such throttle or stop valves are operable in the event of required closure under contingency situations.
In one typical valve arrangement for a turbine installation, each of two single-ended steam chests supply turbine steam through four governor valves. A single throttle valve supplies steam to each steam chest from the plant steam source. An electrohydraulic controller positions the eight governor valves for variation in turbine load while the two throttle valves are held in their wide-open position.
To test a throttle valve associated with one of the steam chests, it is necessary first to close the governor valves downstream from that throttle valve, then make a test closure of the throttle valve, and reopen the throttle valve; and finally reopen the associated governor valves. The same procedure is repeated for the second steam chest. Governor valve closure prior to throttle valve closure is necessary to aviod a high steam pressure drop across the test throttle valve when it is closed since throttle valves typically are not reopenable against high steam pressure. On the other hand, governor valves can be reopened against high steam pressure since they typically operate with balanced valve plug forces.
In initiating, performing, or terminating such throttle or stop valve tests, it is desirable that there be no disturbance to the required load or total steam flow to the turbine. Also, it is desirable that such a test may be performed while the turbine is operating in either the single valve mode or the sequential valve mode without the necessity of turbine operational feedback loops being in service. Of course, depending on the number of throttle or stop valves associated with a particular turbine installation, the load on the turbine must be below a predetermined maximum such that the total desired flow of steam can be accommodated by parallel connected valves not under test. If required, the predetermined maximum load would be met by the taking up of the additional load by another turbine prior to the test.
In conventional feedback analog turbine control systems, governor valve closure for the test procedure is accomplished by the application of an electrical test bias signal to the electrohydraulic controllers for those governor valves associated with one steam chest. As the governor valves are closed, an impulse pressure feedback control automatically causes the remaining governor valves in the other steam chest to open wider and meet the load demand as the governor valves involved in the throttle valve test are closed. Alternatively, a manual control input can be used to raise the load demand signal artificially high so that the remaining governor valves more or less provide the load actually desired. Once the throttle valve test is completed, the test bias is removed from the associated governor valve controllers and turbine load operation is returned to normal. Thus, throttle valve testing can typically be performed with existing electrohydraulic feedback control systems substantially without disturbing existing load. Further, throttle valve testing can typically be initiated in either the sequential or the single valve mode of governor valve operation. Following a test, the governor valves in the one steam chest are reopened, and all governor valves smoothly move under feedback control to the positions required to satisfy load demand in the pretest governor valve mode.
A throttle or stop valve test system is proposed in copending U.S. Patent Application Ser. No. 388,534, filed Aug. 15, 1973 by Uri G. Ronnen and Francesco Lardi for a digital electrohydraulic turbine control system which generates a feedforward representation of desired total steam turbine flow. As a part of this digital electrohydraulic system, a valve positioning system is governed by the total steam flow representation to calculate dynamically a nonlinear characterization of steam flow vs. valve lift position for each of the governor valves in a selected single or sequential mode of valve operation. This valve positioning system functions to transfer from one valve mode to another without effectively disturbing the total steam flow to the turbine. However, the valve test system of the copending application include a linear bypass system in the program digital computer which functions to control the governor valves which are taking up the steam flow at times when the governor valves in the test path are being closed preparatory to such throttle valve test, and the turbine operative feedback loops are required to be in service. The special bypass linear demand function or subroutine for testing is rendered inactive when the nonlinear valve system or subroutine returns to its normal operation after a valve test. This nonlinear valve positioning routine or system is provided with a tracking function and the necessary inputs for generation of feedforward valve position demand representation which conform to existing valve position at the termination of the test. Thus, the transfer from normal automatic operation to test operation and back to normal operation is accomplished bumplessly in the system of the copending application.
Although the stop or throttle valve test system in the copending application is effective, and does not disturb the load on the turbine, it is desirable to be able to provide such a system that uses a feedforward signal and does not require feedback loops to be in service to achieve a smooth valve test, or require the additional valve test hardware. It is also desirable to be able to initiate the valve test during either the single or sequential mode of operation.